Embryology

Cleavage the first step in the development of any multi-celled organism. Its purpose is to convert a single celled zygote into a multi-celled embryo through mitosis. Through this process the blastula is formed by mitosis of the zygote. The blastula is a basically a ball of cells that is surrounding the blastocoel (the fluid-filled center). Both the blastula and the gastrula are part of the early embryonic stages of development, but they are different from each other. The blastula is the early stage of a dividing embryo, and the gastrula is the early stage of development in which the cells split into three embryonic germ layers. To make this more clear, what happens is the following: Once the blastula is formed, then the blastopore (opening in the blastula) starts to cave in. and the cells begin to migrate to specific locations. This leads to the formation of the ectoderm, mesoderm and endoderm. This process is known as gastrulation. During the grastula stage, the archenteron also forms. The archenteron is a cavity that is the beginnings of a digestive tract.

During grastualtion, the three layers that are formed are the outer, inner and middle cell layers.

1. Ectoderm: This is the outer cell layer. It forms tissues associated with outer layers. This includes the skin, hair, sweat glands, and epithelium. The brain and nervous system can also develop from this cell layer
2. Mesoderm: This is the middle cell layer. This forms structures involved in movement and support. This includes body muscle, cartilage, bone, blood, and other connective tissue. Reproductive system organs and kidneys are also formed from the mesoderm layer.
3. Endoderm: This is the inner cell layer. It forms organs and tissue involved with digestive and respiratory systems. Many endocrine glands (thyroid and parathyroid) are formed by the endoderm. The liver, gallbladder, and pancreas are also formed by the endoderm layer.

• When organogenesis occurs, this is when the formation of organs happens. These organs are formed by each layer of cells, each responsible for the formation of different organs.

An acrosomal reaction occurs when the sperm head makes contact with the jelly from an egg. This happens when the acrosome of the sperm (cap-like structure over the anterior half of the sperms head) penetrates the hard shell of the egg.

ACROSOMAL REACTION ON A SEA URCHIN CELL

A cortical reaction is the time after the sperm has entered the egg. The egg membrane then undergoes changes that will prevent any other sperm from entering into the egg.